D&D combat in one page (of really small type)

Initiative

Every round, each person acts once, from highest initiative to lowest. Your initiative is determined by rolling 1d20 + your Dex bonus, and stays the same until the end of the combat.

If some participants are taken by surprise, the other ones get a free round before the surprised ones can act.

Before the very first time you act in a combat, you are "flat-footed"; you don't get a Dex bonus to your armor class and can't make Attacks of Opportunity (see below).

Taking actions

There's lots of confusing stuff in the rules about standard actions, full-round actions, move actions, free actions, etc. Here's what I think is a simpler way to think about it: So you can take one full-round action, or a standard and a move, or two moves, and all of that is along with as many free actions as you like, plus maybe a move of five feet.

Attacks of Opportunity (AoO)

Some things that you do (typically, actions that distract you from defending yourself) provoke an Attack of Opportunity (AoO), which means that nearby opponents get a free attack (although a combatant can only take one AoO per round, even if multiple opponents provoke one). So try to avoid that. Withdraw and Move Five Feet are particularly helpful in avoiding attacks of opportunity while remaining mobile.

Actions

I've left out actions that are obscure or high-level. Actions that provoke 'Attacks of Opportunity' are marked with a star.

Resolving a normal attack

Roll a d20 + your Base Attack Bonus + your Str modifier (or Dex if it's a missile weapon). If you beat your target's armor class (AC), you do damage equal to your weapon + your Str modifier (only if it's not a missile weapon).

If you roll a 1, you automatically miss. If you roll a 20, you automatically hit. If you roll in your weapon's threat range (typically just 20 but sometimes more), you may have a critical hit; roll again, and if that's a hit, you do some multiple of your usual damage.

For damage, roll the dice associated with your weapon and add your strength bonus (one and a half times the strength bonus if it's a two-handed weapon, or half of the strength bonus if the weapon is in your off hand). Damage can't go below 1.

Injury and death

A combatant reduced to 0 hit points is disabled (3 action points per turn). At -1 HP, you're dying. Every round, you have a 10% chance of becoming stable; otherwise you lose another hit point. When you reach -10, you die. You can stabilize someone by successfully making a DC 15 Heal check.

Combat modifiers

If the defender is behind physical cover, he gets +4 to his armor class.

If he has concealment (e.g., in darkness, fog, or tall grass), then any successful hit on him has a 20% chance of missing anyway.

If you and an ally are on opposite sides of a defender, he is flanked and you both get +2 to hit.

Special attacks

You can make other kinds of attacks than just swinging or shooting at someone, if you're feeling creative: